Twenty cases of the distinctive tumor-like lesion of the ovary, pregnancy luteoma, are described, with emphasis on pathologic features. The masses occurred in patients from 15 to 44 years of age and were typically incidental findings at or near term. Four patients experienced androgenic manifestations. The luteoma was documented to be bilateral in only 4 cases, but the opposite ovary was usually not evaluated pathologically. They ranged up to 15 cm. Sectioning typically showed multiple nodules or a single discrete mass with a multinodular sectioned surface and a soft bulging appearance. Most lesions were brown, but a few were black or yellow at least focally. Foci of hemorrhage were common. The most common microscopic appearance was a diffuse growth, but it was punctuated in about three quarters of the cases by follicle-like spaces often containing colloid-like secretion. These spaces are much more characteristic of the pregnancy luteoma than of the lesion most often in the differential diagnosis, a steroid cell tumor, and accordingly may be of diagnostic aid. However, they may cause confusion, as follicle-like spaces are a nonspecific feature of a number of neoplasms involving the ovary and potentially in the differential diagnosis. Awareness of the clinical background and frequency of finding follicle-like spaces in pregnancy luteoma are important diagnostically, and standard immunohistochemical stains will aid should they be warranted.